Comprehensive study of carbon and oxygen isotopic compositions, trace element abundances, and cathodoluminescence intensities of calcite in the Murchison CM chondrite

The O isotopic ratios, trace element concentrations, and CL characteristics provide no evidence for C-isotope evolution in fluids from a single C reservoir by Rayleigh-type isotope fractionation (i.e., removal of C-bearing gaseous species). Also, it seems difficult to explain the O and C isotopic compositions of the two types of calcites by their formation at different temperatures from a single fluid. Instead, the δ13C variation suggests the presence of at least two C reservoirs with different isotopic ratios in the aqueous fluids from which the calcites precipitated. The C reservoirs with lower δ13C values are likely to be organic matter. The same holds for the C reservoirs with higher δ13C values which might have significant contributions from the 13C-enriched grains identified in meteoritic insoluble organic matter. Thermodynamic calculations show that calcite with lower Fe concentrations formed under more reduced conditions than calcite with higher Fe concentrations. If this is the case, the 13C-rich organic grains may have been destroyed and dissolved in the fluids under more reduced conditions than other organic components. The fact that the two types of calcites were found in different domains in the same thin section suggests that microenvironments with diverse physicochemical conditions such as redox states were present at scales of 100's μm.